On 4/27/12 9:04 PM, Daniel Peebles wrote:
Hi all,
Currently Data.Map has a Monoid instance, but it's rather lossy and not as general as it could be:
instance (Ord k) => Monoid (Map k v) where mempty = empty mappend = union mconcat = unions
The instance would be much nicer if it required a Monoid on v and used unionWith mappend instead of just union.
I'm inclined to agree as well.
I realize that changing instances could break code, but I'd be curious to see how many people even use the current monoid instance. Does anyone have any system for testing hypotheses like this (by typechecking a large randomized chunk of hackage or something)?
The thing I'd be more worried about is silent changes to semantics. At least if it doesn't typecheck then you know something went wrong; but there are lots of monoids and so it's very likely to typecheck but to alter semantics. When I've been curious about things like this before, I've just used grep on my local copy of Hackage. But that only works for things with relatively unique names; it'd be completely unhelpful for something type-directed like this. You could try removing the instance entirely and then see how much of Hackage you can get to compile. That way you'll detect all uses, not just the uses for non-monoidal value types. You should be able to get your hands on one of the build-all-of-Hackage scripts people've used for this sort of thing in the past. -- Live well, ~wren